7,088 research outputs found
Quantum dense coding over Bloch channels
Dynamics of coded information over Bloch channels is investigated for
different values of the channel's parameters. We show that, the suppressing of
the travelling coded information over Bloch channel can be increased by
decreasing the equilibrium absolute value of information carrier and
consequently decreasing the distilled information by eavesdropper. The amount
of decoded information can be improved by increasing the equilibrium values of
the two qubits and decreasing the ratio between longitudinal and transverse
relaxation times. The robustness of coded information in maximum and partial
entangled states is discussed. It is shown that the maximum entangled states
are more robust than the partial entangled state over this type of channels
Unified Treatment of Heterodyne Detection: the Shapiro-Wagner and Caves Frameworks
A comparative study is performed on two heterodyne systems of photon
detectors expressed in terms of a signal annihilation operator and an image
band creation operator called Shapiro-Wagner and Caves' frame, respectively.
This approach is based on the introduction of a convenient operator
which allows a unified formulation of both cases. For the Shapiro-Wagner
scheme, where , quantum phase and amplitude
are exactly defined in the context of relative number state (RNS)
representation, while a procedure is devised to handle suitably and in a
consistent way Caves' framework, characterized by , within the approximate simultaneous measurements of
noncommuting variables. In such a case RNS phase and amplitude make sense only
approximately.Comment: 25 pages. Just very minor editorial cosmetic change
Structural phase transitions in epitaxial perovskite films
Three different film systems have been systematically investigated to
understand the effects of strain and substrate constraint on the phase
transitions of perovskite films. In SrTiO films, the phase transition
temperature T was determined by monitoring the superlattice peaks
associated with rotations of TiO octahedra. It is found that T depends
on both SrTiO film thickness and SrRuO buffer layer thickness. However,
lattice parameter measurements showed no sign of the phase transitions,
indicating that the tetragonality of the SrTiO unit cells was no longer a
good order parameter. This signals a change in the nature of this phase
transition, the internal degree of freedom is decoupled from the external
degree of freedom. The phase transitions occur even without lattice relaxation
through domain formation. In NdNiO thin films, it is found that the
in-plane lattice parameters were clamped by the substrate, while out-of-plane
lattice constant varied to accommodate the volume change across the phase
transition. This shows that substrate constraint is an important parameter for
epitaxial film systems, and is responsible for the suppression of external
structural change in SrTiO and NdNiO films. However, in SrRuO films
we observed domain formation at elevated temperature through x-ray reciprocal
space mapping. This indicated that internal strain energy within films also
played an important role, and may dominate in some film systems. The final
strain states within epitaxial films were the result of competition between
multiple mechanisms and may not be described by a single parameter.Comment: REVTeX4, 14 figure
The chromatic Point Spread Function of weak lensing measurement in Chinese Space Station survey Telescope
The weak gravitational lensing is a powerful tool in modern cosmology. To
accurately measure the weak lensing signal, one has to control the systematic
bias to a small level. One of the most difficult problems is how to correct the
smearing effect of the Point Spread Function (PSF) on the shape of the
galaxies. The chromaticity of PSF for a broad-band observation can lead to new
subtle effects. Since the PSF is wavelength dependent and the spectrum energy
distributions between stars and galaxies are different, the effective PSF
measured from the star images will be different from that smears the galaxies.
Such a bias is called colour bias. We estimate it in the optical bands of the
Chinese Space Station Survey Telescope from simulated PSFs, and show the
dependence on the colour and redshift of the galaxies. Moreover, due to the
spatial variation of spectra over the galaxy image, there exists another
higher-order bias, colour gradient bias. Our results show that both colour bias
and colour gradient bias are generally below percent in CSST. Only for
small-size galaxies, one needs to be careful about the colour gradient bias in
the weak lensing analysis using CSST data
A highly stable atomic vector magnetometer based on free spin precession
We present a magnetometer based on optically pumped Cs atoms that measures
the magnitude and direction of a 1 T magnetic field. Multiple circularly
polarized laser beams were used to probe the free spin precession of the Cs
atoms. The design was optimized for long-time stability and achieves a scalar
resolution better than 300 fT for integration times ranging from 80 ms to 1000
s. The best scalar resolution of less than 80 fT was reached with integration
times of 1.6 to 6 s. We were able to measure the magnetic field direction with
a resolution better than 10 rad for integration times from 10 s up to 2000
s
A phonon scattering assisted injection and extraction based terahertz quantum cascade laser
A novel lasing scheme for terahertz quantum cascade lasers, based on
consecutive phonon-photon-phonon emissions per module, is proposed and
experimentally demonstrated. The charge transport of the proposed structure is
modeled using a rate equation formalism. An optimization code based on a
genetic algorithm was developed to find a four-well design in the
material system that maximizes the product
of population inversion and oscillator strength at 150 K. The fabricated
devices using Au double-metal waveguides show lasing at 3.2 THz up to 138 K.
The electrical characteristics display no sign of differential resistance drop
at lasing threshold, which suggests - thanks to the rate equation model - a
slow depopulation rate of the lower lasing state, a hypothesis confirmed by
non-equilibrium Green's function calculations.Comment: 11 pages, 10 figure
Temperature and Strain Sensors Based on Integration of Tilted Fiber Bragg Gratings With a Free Spectral Range Matched Interrogation System
In this work, a tilted fiber Bragg grating (TFBG) is integrated with a free spectral range matched interrogation (FSRMI) system for temperature and strain sensing applications. The experimental data show that the peak wavelengths of the core and cladding modes of the TFBG shift linearly to longer wavelength with the increase of temperature and strain. Since the FSRMI system allows simultaneous demodulation of multiple wavelengths, the sensing system can be extended by integrating a eta-channel FSRMI with eta TFBGs for multipoint temperature and strain sensing with high precision and fast detection rate
Electrically switching transverse modes in high power THz quantum cascade lasers.
The design and fabrication of a high power THz quantum cascade laser (QCL), with electrically controllable transverse mode is presented. The switching of the beam pattern results in dynamic beam switching using a symmetric side current injection scheme. The angular-resolved L-I curves measurements, near-field and far-field patterns and angular-resolved lasing spectra are presented. The measurement results confirm that the quasi-TM(01) transverse mode lases first and dominates the lasing operation at lower current injection, while the quasi-TM(00) mode lases at a higher threshold current density and becomes dominant at high current injection. The near-field and far-field measurements confirm that the lasing THz beam is maneuvered by 25 degrees in emission angle, when the current density changes from 1.9 kA/cm(2) to 2.3 kA/cm(2). A two-dimension (2D) current and mode calculation provides a simple model to explain the behavior of each mode under different bias conditions
A phonon scattering assisted injection and extraction based terahertz quantum cascade laser
A novel lasing scheme for terahertz quantum cascade lasers, based on
consecutive phonon-photon-phonon emissions per module, is proposed and
experimentally demonstrated. The charge transport of the proposed structure is
modeled using a rate equation formalism. An optimization code based on a
genetic algorithm was developed to find a four-well design in the
material system that maximizes the product
of population inversion and oscillator strength at 150 K. The fabricated
devices using Au double-metal waveguides show lasing at 3.2 THz up to 138 K.
The electrical characteristics display no sign of differential resistance drop
at lasing threshold, which suggests - thanks to the rate equation model - a
slow depopulation rate of the lower lasing state, a hypothesis confirmed by
non-equilibrium Green's function calculations.Comment: 11 pages, 10 figure
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